Technology to analyze nucleotide base sequences of DNA is not simply limited to the academic research field, but is applied in fields ranging from medicine to drug discovery and criminal forensics, and there is increasing interest in developments in this technology.
Conventional DNA sequencers that have been developed hitherto employ optical measuring techniques to identify fluorescent markers added to nucleotides, rather than directly identifying the nucleotides themselves. In order to attach a marker to a nucleotide, it is necessary to use PCR to chemically modify the nucleotide. This process not only requires a large number of reagents, but is also very time consuming. Serious funding and time are, therefore, required in order to perform DNA sequencing.
Against this backdrop, over the past several decades there has been striking progress in DNA sequencing technology based on single molecules as a step towards individual genome sequencing. One recent example of an outstanding achievement brought about by this progress is a detector employing nanoscale pores (referred to below as nanopores) of chemically designed α-hemolysin (see Non-Patent Documents 1 to 5).
This detector enables sequencing to be performed on a single DNA molecule by detecting temporary ion current blocking that occurs when an oligomer of a single strand of DNA passes through an embedded biological nanopore in cyclodextrin (see Non-Patent Documents 6, 7). Such strong and solid state nanopores capable of being structured are attracting attention, and play a role as platforms for researching the dynamics of single biomolecules passing through pores (see Non-Patent Documents 1, 2, 8 to 11). However, there are issues in that DNA sequencing based on ion currents employing the above detector is (1) limited in pore size selection, and is (2) an unstable system, such that there is no real prospect of application of sequencers employing biological nanopores. Research into DNA sequencing with a resolution of one molecule based on an ion current is still ongoing (see Non-Patent Documents 3, 4).
A theory of sequencing based on transverse electron transport has been proposed as an alternative to ion current based DNA sequencing. As illustrated in FIG. 21, this theory is based on a principle of detecting transverse conductivity distinct to each nucleotide when a nucleotide passes through a nanoscale space between a pair of electrodes (this conductivity is related to differences in gaps between the HOMO and the LUMO of each nucleotide). Specifically, during passage of single DNA through a nanopore, a tunnel current occurs through each nucleotide between a pair of electrodes provided at the nanopore edges with a nanoscale inter-electrode distance (referred to below as the “nanoelectrode pair”). It is thought that by measuring the current value of the tunnel current, direct reading of the nucleotide sequence based on the current value is possible without marking.
It is anticipated that were such transverse electron transport based sequencing to be performed, it would be possible to directly read the nucleotide sequence of a single DNA molecule at an extremely high sequencing speed in excess of 400 kilobases per hour (see Non-Patent Documents 5, 12, 13). Based on such theoretical anticipations, a number of groups have developed systems in order to demonstrate these predictions by embedding nanoelectrode pairs in fluid channels with sizes in micrometers or nanometers (see Non-Patent Documents 16 to 19).
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